Physics Department

Olber's Paradox

Olber's Paradox

An active question in astrophysics goes this way. Assuming the universe is infinitely large and would than contain an infinite amount of roughly uniformly distributed stars, then should not the night sky be blazing with light from these stars. That is, even if the farther stars are fainter, their number increases with distance, thus there should be an enormous amount of the star light reaching Earth. The reality is that the night sky is relative dark. What do you think?

A. The Universe really is not infinite and there is not an infinite number of stars. The size of the Universe is small enough that the number of stars is not enough to light the night sky.

B. The farther a star is away, the faster it is receding (Hubble's principle), therefore, the light from the farthest stars is red shifted (a Doppler shift for light) below the visible region. Therefore there is not enough visible light to light the night sky.

C. The lifetime of a star is about 1010 years. The years for the Universe to reach thermal equilibrium is about 1024 years. Many stars have been born, lived, and died already. Basically at any one time there are not enough stars active to fill the space of the Universe with enough radiation to light the night sky

D. Space and the Universe is filled with many clouds of 'space dust'. These clouds absorb visible light. Thus enough light is not available to light the night sky.